The precise physical mechanism for the formation of the quasi-periodic oscillations is currently unknown but some features suggest that they are a hydrodynamical phenomenon, rather than a manifestation of kinematic effects in the accretion disk like the Doppler modulation of fluxes from isolated hot spots. Such an example is the discovery of the correlation between the low-and high-frequency QPOs, causing that their ratio remains stable among various X-ray sources [1]}-[2]}. This motivated the development of disk seismological models which explain the QPOs by means of certain trapped modes of the disk oscillations [3]}-[4]}. It was further observed that the high-frequency QPOs scale inversely with the mass of the compact object and that the twin peaks follow a constant \(3:2\)  ratio. Thus, we got indications that the high-frequency QPOs are caused by relativistic effects so that they represent a suitable probe of the background spacetime. The characteristic frequency ratio of two integers further suggests that the source of the quasi-periodic oscillations can be some non-linear resonance process taking place in the inner disk.
